**Core Concept**
Haemoglobin is a vital protein in red blood cells responsible for binding and transporting oxygen (O2) from the lungs to the body's tissues. Besides oxygen, haemoglobin can also bind to various other molecules, including carbon dioxide (CO2), nitric oxide (NO), and carbon monoxide (CO). This binding ability is crucial for maintaining proper oxygenation and acid-base balance in the body.

**Why the Correct Answer is Right**
Haemoglobin binds to O2 through a reversible reaction with the iron (Fe2+) in the haem group. This binding is facilitated by the Bohr effect, where the binding of CO2 to haemoglobin increases the affinity of haemoglobin for O2. However, sulphur dioxide (SO2) does not bind to haemoglobin in a similar manner. Instead, SO2 is a toxic gas that can cause oxidative stress and damage to haemoglobin, leading to methemoglobinemia. The presence of SO2 in the blood can also lead to the formation of sulphhemoglobin, a condition where haemoglobin is bound to sulphur instead of oxygen.

**Why Each Wrong Option is Incorrect**
**Option A:** O2 is the primary ligand that binds to haemoglobin, and it is essential for the proper functioning of the respiratory system.
**Option B:** CO2 binds to haemoglobin, forming carbaminohaemoglobin, which is an important mechanism for transporting CO2 from the tissues to the lungs.
**Option D:** NO binds to haemoglobin, forming methaemoglobin, which can be an important mechanism for regulating blood pressure and vasodilation.

**Clinical Pearl / High-Yield Fact**
It is essential to note that sulphur dioxide (SO2) can cause oxidative stress and damage to haemoglobin, leading to conditions such as methemoglobinemia. This is a critical consideration in patients with pre-existing respiratory conditions, such as chronic obstructive pulmonary disease (COPD), who may be more susceptible to the toxic effects of SO2.

**✓ Correct Answer: C. SO2**